JP4729423B2 - Optical interferometer - Google Patents

Description

  The present invention relates to an optical interferometer. Specifically, the present invention relates to an optical interferometer that measures a change in thickness of a measurement object.

An optical interferometer is known as an apparatus for measuring unevenness of a test surface (for example, Patent Document 1).
FIG. 8 is a diagram illustrating a Michelson-type optical interferometer 10.
In FIG. 8, the light emitted from the light source 11 is converted into a parallel light flux by the collimating lens system 12, then passes through the half mirror 13, is focused by the lens 14 on the way, and is irradiated onto the workpiece W. The light applied to the workpiece W is reflected by the workpiece W to become object light La, is reflected by the half mirror 13 and enters the interference fringe acquisition unit. Here, a beam splitter 16 and a reference mirror 17 are disposed between the lens 14 and the workpiece W. Of the light that has passed through the lens 14, the light that has passed through the beam splitter 16 is irradiated onto the workpiece W to become object light La, whereas the light reflected by the beam splitter 16 is incident on the reference mirror 17. It is reflected to become reference light Lb.
The reference light Lb is reflected again by the beam splitter 16 and enters the interference fringe acquisition unit 15 via the half mirror 13. The interference fringe acquisition unit 15 acquires interference fringes generated by interference between the object light La and the reference light Lb by imaging with a CCD camera (not shown). And the unevenness | corrugation of a workpiece | work surface can be calculated | required by analyzing the fringe space | interval etc. of an interference fringe.

  Thus, the unevenness of the workpiece surface can be obtained using the optical interferometer 10, and there is a further demand for knowing the thickness change of the workpiece W. For example, it is necessary to know a change in the thickness of a semiconductor wafer. In this way, when it is desired to know the thickness change of the workpiece W, for example, it is conceivable to alternately measure the front and back surfaces of the workpiece W by the optical interferometer 10 of FIG. That is, the front surface of the workpiece surface is measured by the optical interferometer 10 shown in FIG. Furthermore, the workpiece W is turned over and the unevenness on the back surface of the workpiece is measured. And it is thought that the thickness change of a workpiece | work can be known by synthesize | combining the shape data of the workpiece | work W front surface and a workpiece back surface.

JP-A-8-35883

By turning the workpiece W upside down and measuring the workpiece front surface Sa and the workpiece back surface Sb, it is possible to obtain the unevenness data of the front surface Sa and the back surface Sb.
However, the thickness change of the workpiece W cannot be obtained unless the points of the surface data Da and the points of the back surface data Db can be matched when combining the measurement data. That is, even if the front surface data Da and the back surface data Db are obtained as shown in FIG. 9, if a point on the same axis cannot be specified by the front surface data Da and the back surface data Db, The thickness change cannot be obtained.
In FIG. 9, it is assumed that the point on the same axis in the back surface data is the point A2 with respect to the front surface data at the point A1. If the front surface data Da of the point A1 and the back surface data Db of the point A2 are made to correspond, data corresponding to the thickness of the workpiece W can be obtained. Therefore, the thickness change of the workpiece W can be obtained by making the data on the same axis correspond to each other by the front surface data Da and the back surface data Db.
However, when a point on the same axis as the point A1 cannot be accurately specified, if the point B2 that is a point deviated from the axis is associated with the point A1, the thickness change of the workpiece W can be obtained. Can not.

Alternatively, as shown in FIG. 10, it is conceivable to arrange the optical interferometer 10 on the front surface side and the work back surface side with the work W interposed therebetween. An optical flat (not shown) with coaxial points associated with the front and back surfaces is prepared in advance, and both front and back surfaces of the optical flat are measured before measuring the workpiece W. It is considered that the points should be associated with Da and the back side data Db.
However, it is difficult in the first place to prepare an optical flat in which both front and back surfaces are flat and undulate with high accuracy and the front and back surfaces are associated with each other with high accuracy.
Therefore, even if the two optical interferometers 10 are arranged with the workpiece W sandwiched in this way, it is possible to obtain the change in the thickness of the workpiece W by matching the front surface data Da and the back surface data Db on the same axis. Have difficulty.

Further, the front surface side optical interferometer 10 and the back surface side optical interferometer 10 each include a reference mirror 17.
Here, the unevenness change obtained by the interference fringes is a difference in unevenness and inclination between the reference mirror 17 and the workpiece surface.
Therefore, if there is a difference in the unevenness and inclination of the reference mirror 17 between the optical interferometer 10 on the front surface side and the optical interferometer 10 on the back surface side, the front surface data Da and the back surface data Db are obtained. Even if they are combined, only a value obtained by adding the unevenness and inclination difference between the reference mirrors to the thickness change of the workpiece W is obtained. That is, it is not possible to determine only the thickness change of the workpiece.
Of course, it is conceivable to prepare an optical flat and obtain the unevenness and inclination of the reference mirror 17 in advance. However, as described above, it is difficult to prepare a high-precision optical flat in the first place.

  Due to the above problems, it was not possible to meet the needs for changing the thickness of the workpiece.

  An object of the present invention is to provide an optical interferometer that can determine a change in the thickness of a workpiece.

  The optical interferometer of the present invention is an optical interferometer that measures a change in the thickness of a workpiece, and is disposed on the front surface side of the workpiece and the back surface side of the workpiece. A second optical interferometer, wherein the first optical interferometer and the second optical interferometer are disposed between a light emitting unit, the light emitting unit, and the workpiece, and have a reference surface. In addition, polarization separation means for making the polarization direction of the reference light, which is the reflected light from the reference surface, and the object light, which is the reflected light from the workpiece, perpendicular to each other, and interference fringes obtained by causing the reference light and the object light to interfere with each other are obtained. An interference fringe acquisition unit, and the polarization separation unit of the first optical interferometer and the polarization separation unit of the second optical interferometer are arranged in a state in which the polarization axes are not parallel, and no workpiece is arranged. In the state, the reference surface of the polarization separation means of the first optical interferometer is The light from the second optical interferometer is reflected to generate object light, and the reference surface of the polarization separation means of the second optical interferometer reflects the light from the first optical interferometer to generate an object light. It is characterized by generating light.

In such a configuration, when the workpiece is set, interference fringes reflecting the shape of the workpiece front surface are obtained by the first optical interferometer, and the shape of the workpiece back surface is obtained by the second optical interferometer. Is obtained.
First, the light emitted from the light emitting unit enters the polarization separation means. A part of the light incident on the polarization separation means is reflected by the reference surface of the polarization separation means to become reference light. Further, the remainder of the light incident on the polarization separation means is irradiated onto the work from the polarization separation means and then reflected by the work to become object light. At this time, the polarization directions of the reference light and the object light are perpendicular to each other and become non-interfering lights that do not interfere with each other.
The reference light and the object light enter the interference fringe acquisition unit. For example, when the reference light and the object light are caused to interfere by passing through the polarizing plate, the interference fringe acquisition unit acquires the interference fringes. In this way, an interference fringe reflecting the shape of the workpiece front surface is obtained by the first optical interferometer, and an interference fringe reflecting the shape of the workpiece back surface is obtained by the second optical interferometer.

Next, a state where no workpiece is set will be described.
When no work is set, the light from the first optical interferometer reaches the second optical interferometer because there is no work. A part of the light incident on the polarization separation means of the first optical interferometer is reflected by the reference surface to become reference light, which is the same as when the work is set. On the other hand, of the light incident on the polarization separation means of the first optical interferometer, the light to be irradiated on the work reaches the second optical interferometer. Then, this light is reflected by the reference surface of the polarization separation means of the second optical interferometer and returned to the first optical interferometer.
The light returned from the second optical interferometer to the first optical interferometer enters the interference fringe acquisition unit as object light. Then, in the first optical interferometer, interference fringes in which the object light and the reference light interfere are acquired. Even in the second optical interferometer, since there is no work, the light from the second optical interferometer reaches the first optical interferometer, and the object light by the reference surface included in the polarization separation means of the first optical interferometer and the second optical interferometer Interference fringes in which the reference light from the reference surface of the polarization separation means of the interferometer interferes are acquired. That is, in a state where no workpiece is set, interference fringes in which reflected light from the reference surfaces of the two polarization separation means interfere with each other in both the first optical interferometer and the second optical interferometer. Therefore, when the workpiece is not set, the interference fringes acquired by the first optical interferometer and the second optical interferometer are the same.

According to the present invention, the workpiece thickness change can be accurately obtained by combining the workpiece front surface data and the workpiece back surface data after associating the points on the same axis. .
Work surface data is obtained by the first optical interferometer, and work back surface data is obtained by the second optical interferometer. The work front surface data and the work back surface data are synthesized to change the thickness of the work. In order to obtain the point, the points on the same axis must be associated with the workpiece front surface data and the workpiece back surface data. This is because, even if the data is synthesized with the front and back surfaces being shifted in axis, the thickness change of the workpiece is not correctly obtained.

In this regard, in the present invention, when the workpiece is not set, the interference fringes acquired by the first optical interferometer and the second optical interferometer are the same, and therefore the first optical interferometer and the second optical interference are the same. By associating the same point of the data acquired with the meter, the coaxial point on the data can be associated. For example, when the position of the interference fringe acquisition unit is shifted between the first optical interferometer and the second optical interferometer, the position in the imaging screen is shifted even if the same interference fringe is imaged. Since the first optical interferometer and the second optical interferometer can obtain the pixel correspondence relationship between the coaxial points in the imaging screen.
As described above, after the pixel correspondence relationship in the imaging screen is obtained by the first optical interferometer and the second optical interferometer, the work is set and the work is performed by the first optical interferometer and the second optical interferometer. The surface data and the back surface data of the work are acquired. Then, the thickness change of the workpiece can be obtained by synthesizing the workpiece front surface data and the workpiece back surface data in consideration of the correspondence between the already determined points on the same axis.

Further, according to the present invention, it is possible to obtain a change in the thickness of the workpiece with high accuracy regardless of the difference between the unevenness of the reference surface of the first optical interferometer and the unevenness of the reference surface of the second optical interferometer. . When the workpiece front surface data is acquired by the first optical interferometer in the state where the workpiece is set, this interference fringe is a fringe based on a gap change between the reference surface of the first optical interferometer and the workpiece front surface. It is. Similarly, when the workpiece back surface data is acquired by the second optical interferometer, the interference fringes are fringes based on a gap change between the reference surface of the second optical interferometer and the workpiece back surface.
For this reason, if there is a difference in the unevenness of the reference mirror between the first optical interferometer and the second optical interferometer, the two reference mirrors can change the thickness of the workpiece even if the front surface data and the back surface data are combined. It is only possible to obtain a value obtained by adding the unevenness difference of the workpieces, and it is not possible to obtain only a change in the thickness of the workpiece.

  In this regard, in the present invention, when no workpiece is set, interference fringes in which the reflected light from the reference surface of the first optical interferometer interferes with the reflected light from the reference surface of the second optical interferometer can be obtained. Therefore, the unevenness difference between the two reference surfaces can be obtained from the interference fringes obtained without setting the workpiece. Therefore, after combining the workpiece front surface data and the workpiece back surface data, the thickness change of the workpiece can be obtained with high accuracy by further reducing the unevenness difference between the two reference surfaces.

  In the present invention, the polarization separation means is a wire grid type polarizing plate having a plurality of wires arranged in parallel to each other and reflecting a component parallel to the wire and transmitting a component perpendicular to the wire. In the first optical interferometer and the second optical interferometer, the wire arrangement direction of the wire grid type polarizing plate is preferably non-parallel.

In such a configuration, first, a state where a workpiece is set will be described.
The light emitted from the light emitting unit enters the wire grid. Of the light incident on the wire grid, the component parallel to the wire is reflected by the wire grid. Here, the reflection surface of the wire grid serves as a reference surface, and the light reflected on the reference surface serves as reference light. Moreover, the component perpendicular | vertical to a wire among the light which injected into the wire grid passes through a wire grid, and is irradiated to a workpiece | work. Then, it is reflected by the work and becomes object light.
Here, since the polarization directions of the reference light and the object light are perpendicular, they are non-interfering light beams that do not interfere with each other, but an interference fringe obtained by causing the reference light and the object light to interfere with each other is obtained by the interference fringe obtaining unit.

Next, a state where no workpiece is set will be described.
Of the light incident on the wire grid from the light emitting part, the component parallel to the wire is reflected as the reference light in the same manner as in the work set state. Of the light incident on the wire grid from the light emitting part, the component perpendicular to the wire passes through the wire grid. The light that has passed through the wire grid reaches the wire grid on the opposite side because there is no workpiece.
Here, the wire arrangement direction of the wire grid is non-parallel between the first optical interferometer and the second optical interferometer.
When the direction of the wire is parallel between the wire grid of the first optical interferometer and the wire grid of the second optical interferometer, the light passing through the wire grid of the first optical interferometer is also the wire grid of the second optical interferometer. Will pass.

  On the other hand, in the present invention, the wire arrangement direction of the wire grid is non-parallel between the first optical interferometer and the second optical interferometer, so that, for example, the light passing through the wire grid of the first optical interferometer is the first When incident on the wire grid of the two optical interferometer, at least a portion is reflected. Then, the reflected light returns to the first optical interferometer as object light. Interference fringes are acquired by interference between the light returning from the second optical interferometer to the first optical interferometer and the reference light reflected by the wire grid of the first optical interferometer at the interference fringe acquisition unit. Similarly, also in the second optical interferometer, the reference light reflected by the wire grid of the second optical interferometer and the light returning from the first optical interferometer interfere with each other in the interference fringe acquisition unit, resulting in interference fringes. Is acquired.

According to such a configuration, since the polarization axis direction of the wire grid is non-parallel between the first optical interferometer and the second optical interferometer, the wire grid of one of the optical interferometers is not set when the workpiece is not set. At least a portion of the light that has passed is reflected by the wire grid of the other optical interferometer.
Thereby, in the state which does not set a workpiece | work, in both the 1st optical interferometer and the 2nd optical interferometer, the interference fringe by the reflected light from two wire grids can be acquired similarly.
Thus, since the same interference fringes can be acquired by the first optical interferometer and the second optical interferometer, the coaxial points of the data acquired by the first optical interferometer and the second optical interferometer can be associated with each other. .
In addition, since the first optical interferometer and the second optical interferometer can obtain interference fringes in which reflected lights from the respective wire grids interfere with each other, the first optical interferometer and the second optical interferometer The unevenness difference between the reference surfaces can be obtained. Therefore, after combining the workpiece front surface data and the workpiece back surface data, the thickness change of the workpiece can be obtained with high accuracy by further reducing the unevenness difference between the reference surfaces.

  In the present invention, it is preferable that the polarization separation unit of the first optical interferometer and the polarization separation unit of the second optical interferometer are arranged in a state where the polarization axes are orthogonal to each other.

A case where a workpiece is not set in such a configuration will be described.
Of the light incident on the polarization separation means from the light emitting part, the light of the polarization component parallel to the polarization axis passes, and the light of the polarization component orthogonal to the polarization axis is reflected.
Here, in a state in which the workpiece is not set, for example, light that has passed through the polarization separation unit of the first optical interferometer enters the polarization separation unit of the second optical interferometer. Then, since the polarization axis directions of the polarization separation means of the first optical interferometer and the polarization separation means of the second optical interferometer are orthogonal, the light that has passed through the polarization separation means of the first optical interferometer All are reflected by the polarization separation means of the two optical interferometer. And the interference fringe which made the 1st optical interferometer and the 2nd optical interferometer interfere with each other reflected light from each polarization separation means can be obtained.

  According to such a configuration, since the polarization axes of the polarization separating means are orthogonal to each other between the first optical interferometer and the second optical interferometer, when one workpiece is not set, The light that has passed through the polarization separation means can be reflected 100% by the polarization separation means of the other optical interferometer. Therefore, in a state where no workpiece is set, the light utilization efficiency is high and clear interference fringes can be obtained.

  In the present invention, the light emitting unit of the first optical interferometer and the light emitting unit of the second optical interferometer may share one light source and light from the one light source to branch the first light source. It is preferable to include a light guide unit that guides the respective optical paths of the first optical interferometer and the second optical interferometer.

  According to such a configuration, since the same light from the same light source is used in the first optical interferometer and the second optical interferometer, the same is used in the first optical interferometer and the second optical interferometer. Wavelength light can be used. By using light of the same wavelength in this way, exactly the same interference fringes can be obtained in the first optical interferometer and the second optical interferometer in a state where no workpiece is set. Therefore, by associating the same points of the same interference fringes between the first optical interferometer and the second optical interferometer, the first optical interferometer and the second optical interferometer accurately associate the coaxial points of the data. be able to.

  Here, as the polarization separation means, for example, a quarter wavelength plate may be used. As polarization separation means, the function of reflecting the light from the light source to generate the reference light, the function of orthogonalizing the polarization direction of the reference light and the object light, and the light coming from the opposite optical interferometer across the workpiece This is because it has only to have a function of reflecting the light and recurring the same path as the incident path. If it has such a function, in a state where no work is arranged, the reference surface of the polarization separation means of the first optical interferometer reflects the light from the second optical interferometer to reflect object light. And the reference surface of the polarization separation means of the second optical interferometer can reflect the light from the first optical interferometer to generate object light.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be illustrated and described with reference to reference numerals attached to respective elements in the drawings.
FIG. 1 is a perspective view showing a configuration of an optical interferometer in the first embodiment of the optical interferometer of the present invention. 2 and 3 are side views of the optical interferometer.

  The optical interferometer 100 includes a first optical interferometer 200 disposed on the front surface Sa side of the workpiece W and a second optical interferometer 300 disposed on the back surface Sb side of the workpiece W. . The first optical interferometer 200 and the second optical interferometer 300 have basically the same configuration, and the first optical interferometer 200 and the second optical interferometer 300 are symmetric with respect to the workpiece W.

  The first optical interferometer 200 includes a light emitting unit 210, a polarization separating unit 220 having a reference plane 223 and having polarization separation characteristics, an interference fringe obtaining unit 230 that obtains interference fringes, and light from the light emitting unit 210. And a beam splitter 240 that transmits light from the polarization separation means side toward the interference fringe acquisition unit 230.

The light emitting unit 210 includes a light source 211 that emits laser light, an optical fiber 212 that guides light, a polarizing plate 214, and a half-wave plate 215. Light emitted from the laser light source 211 is guided by the optical fiber 212 and enters the beam splitter 240.
Here, a polarizing plate 214 and a half-wave plate 215 are disposed between the opening end 213 of the optical fiber 212 and the beam splitter 240, and a light beam having a polarization angle of 45 degrees is formed by the polarizing plate 214 and the half-wave plate 215. The light enters the beam splitter 240.

The polarization separation means 220 is constituted by a wire grid 220.
The wire grid 220 includes a light transmissive plate 221 and a plurality of wires 222 arranged in parallel with each other on one surface of the light transmissive plate 221. The line width of the wires 222 is, for example, about 60 nm, and the distance between the wires is, for example, about 140 nm. In the light incident on the wire grid 220, a component parallel to the wire 222 is reflected by the wire grid 220, and a component perpendicular to the wire 222 is transmitted through the wire grid 220.
Here, the wire grid provided in the first optical interferometer 200 is referred to as a first wire grid 220. In the first wire grid 220, the wires 222 are arranged in the vertical direction. The polarized light reflected by the first wire grid 220 is S wave, and the polarized light that is transmitted through the first wire grid 220 is P wave.
When light is reflected by the first wire grid 220, the reflection surface becomes the reference surface 223.

  The interference fringe acquisition unit 230 includes, for example, a polarizing plate (not shown) having a polarization angle of 45 degrees and a CCD camera (not shown).

  Collimating lenses 251 and 252 are disposed between the beam splitter 240 and the first wire grid 220 and between the beam splitter 240 and the interference fringe acquisition unit 230, respectively.

The second optical interferometer 300 has basically the same configuration as the first optical interferometer 200.
Here, when the second optical interferometer 300 also includes a wire grid, the wire grid of the second optical interferometer 300 is referred to as a second wire grid 320.
The wires 322 of the second wire grid 320 are arranged in the horizontal direction. When viewed along the optical axis, the wire arrangement direction of the second wire grid 320 and the wire arrangement direction of the first wire grid 220 are orthogonal to each other.

  In the second optical interferometer 300 and the first optical interferometer 200, light from one laser light source 211 is branched by an optical fiber 212 and used.

  Note that the second optical interferometer 300 and the first optical interferometer 200 are provided with corresponding parts, but the second optical interferometer 300 is denoted by reference number 300, and the description thereof is omitted.

In the optical interferometer having such a configuration, a light path will be described.
Here, when the optical interferometer 100 is used, there are a measurement mode for measuring the workpiece W and a calibration mode for calibrating the optical interferometer 100.
First, in FIG. 2, the measurement mode when the workpiece W is set will be described. Here, basically, the first optical interferometer 200 and the second optical interferometer 300 are basically the same in the light path, and therefore the first optical interferometer 200 will be described as an example.

  The light emitted from the laser light source 211 is emitted from the opening end 213 of the optical fiber 212 through the optical fiber 212. The light emitted from the optical fiber 212 passes through the polarizing plate 214 to become linearly polarized light, and further passes through the half-wave plate 215 to become a light flux having a polarization angle of 45 degrees. The light that has passed through the half-wave plate 215 is reflected by the beam splitter 240 and is reflected toward the work front surface Sa. The light reflected by the beam splitter 240 becomes a parallel light beam by the collimator lens 251 and enters the first wire grid 220. Then, of the light incident on the first wire grid 220, a component orthogonal to the wire 222 of the first wire grid 220 is transmitted through the first wire grid 220. The light transmitted through the first wire grid 220 is a P wave, and the light transmitted through the first wire grid 220 is irradiated on the work surface and then reflected on the work surface to return to the first wire grid 220 as object light La. And passes through the first wire grid 220.

  In addition, among the light incident on the first wire grid 220 from the collimating lens 251, a component parallel to the wire 222 of the first wire grid 220 is reflected by the first wire grid 220. The light reflected by the first wire grid 220 is an S wave, and the S wave reflected by the first wire grid 220 is the reference light Lb.

Here, the object light (P wave) La that is reflected light from the work front surface Sa and the reference light (S wave) Lb that is reflected light from the first wire grid 220 have orthogonal polarization directions. Therefore, they are combined without interference and enter the beam splitter 240 through the collimator lens 251 as a non-interfering light beam. The light incident on the beam splitter 240 passes through the beam splitter 240 and then enters the interference fringe acquisition unit 230 through the collimator lens 252. Then, the interference fringe acquisition unit 230 acquires an interference fringe in which the non-interfering light beams of the P wave and the S wave interfere.
For example, a non-interfering light beam is caused to interfere by a polarizing plate (not shown) having a polarization angle of 45 degrees, and the interference light is imaged by a CCD camera (not shown).

  Here, the light path in the second optical interferometer 300 is basically the same as the light path in the first optical interferometer 200. However, the second wire grid 320 reflects the P wave and transmits the S wave. Therefore, in the second optical interferometer 300, the object light La reflected by the workpiece W is an S wave, and the reference light Lb reflected by the second wire grid 320 is a P wave.

  The first optical interferometer 200 obtains an interference fringe reflecting the shape of the workpiece front surface Sa, and the second optical interferometer 300 obtains an interference fringe reflecting the shape of the workpiece back surface Sb. The interference fringes reflecting the shape of the workpiece front surface Sa are set as workpiece front surface data Da, and the interference fringes reflecting the shape of the workpiece back surface Sb are set as workpiece back surface data Db. The workpiece front surface data Da and the workpiece back surface data Db are stored in, for example, predetermined memories.

At this time, the interference fringes of the workpiece front surface data Da are caused by the shape difference between the workpiece front surface Sa and the first wire grid reference surface 223, that is, the workpiece front surface Sa and the first wire grid reference. Reflects the change in the gap (d1) with the surface 223.
Similarly, the interference fringes of the workpiece back surface data Db are generated due to a shape difference between the workpiece back surface Sb and the second wire grid 320, that is, the gap between the workpiece back surface Sb and the second wire grid reference surface 323 ( d2) Reflects changes.

Next, a calibration mode for calibrating the optical interferometer will be described.
The calibration mode is executed with the workpiece removed, as shown in FIG. In this case, the light path is basically the same as in the measurement mode. However, in the first optical interferometer 200, the P wave that has passed through the first wire grid 220 is incident on the second wire grid 320. Then, since the second wire grid 320 reflects the P wave, the P wave that has passed through the first wire grid 220 is reflected by the second wire grid 320. The S wave reflected by the first wire grid 220 and the P wave reflected by the second wire grid 320 are incident on the interference fringe acquisition unit 230 of the first optical interferometer 200. Then, the interference fringe acquisition unit 230 of the first optical interferometer 200 acquires the interference fringes.

  In the second optical interferometer 300, the S wave that has passed through the second wire grid 320 is incident on the first wire grid 220. Then, since the first wire grid 220 reflects the S wave, the S wave that has passed through the second wire grid 320 is reflected by the first wire grid 220. The P wave reflected by the second wire grid 320 and the S wave reflected by the first wire grid 220 enter the interference fringe acquisition unit 330 of the second optical interferometer 300. Then, the interference fringe acquisition unit 330 of the second optical interferometer 300 acquires the interference fringes.

  Here, the interference fringes acquired by the first optical interferometer 200 are generated by interference between the S wave reflected by the first wire grid 220 and the P wave reflected by the second wire grid 320. It is a striped pattern. Therefore, when the S wave reflected by the first wire grid 220 is regarded as the reference light and the P wave reflected by the second wire grid 320 is regarded as the object light, the interference fringe acquisition unit 230 of the first optical interferometer 200. The interference fringes obtained in FIG. 3 are caused by the shape difference between the first wire grid 220 and the second wire grid 320, that is, the gap (d0) change between the first wire grid 220 and the second wire grid 320 is reflected. ing.

  Further, the interference fringes acquired by the second optical interferometer 300 are generated by the interference between the P wave reflected by the second wire grid 320 and the S wave reflected by the first wire grid 220. It is a stripe. Therefore, when the P wave reflected by the second wire grid 320 is regarded as the reference light and the S wave reflected by the first wire grid 220 is regarded as the object light, the interference fringe acquisition unit 330 of the second optical interferometer 300 has The interference fringes obtained in this way are caused by the difference in shape between the second wire grid 320 and the first wire grid 220, that is, reflecting the gap (d0) change between the second wire grid 320 and the first wire grid 220. Yes.

  Thus, the interference fringes acquired by the two interference fringe acquisition units 230 and 330 in the calibration mode both reflect the change in the gap (d0) between the first wire grid 220 and the second wire grid 320. Both have the same interference fringes.

Examples of interference fringes imaged by the first optical interferometer 200 and the second optical interferometer 300 are shown in FIG. Although the interference fringes Ca acquired by the first optical interferometer 200 and the interference fringes Cb acquired by the second optical interferometer 300 have the same shape, for example, the attachment positions of the interference fringe acquisition units 230 and 330 In some cases, the position on the imaging screen is shifted due to a shift.
In such a case, the same points of the interference fringes are made to correspond one to one. That is, the coaxial points of the measurement point in the first optical interferometer 200 and the measurement point in the second optical interferometer 300 are made to correspond to each other. For example, the pixels on the imaging screen are associated with each other, and the correspondence is stored in a predetermined memory.

  Further, the interference fringes imaged by the first optical interferometer 200 and the second optical interferometer 300 reflect the change in the gap (d0) between the first wire grid 220 and the second wire grid 320. The interference fringes are stored in a predetermined memory as calibration interference fringes.

Next, the calculation for obtaining the workpiece thickness change will be briefly described.
In the calibration mode, two identical interference fringes based on the gap change between the first wire grid 220 and the second wire grid 320 are acquired by the first optical interferometer 200 and the second optical interferometer 300, and the pixels on the imaging screen We are dealing with it. Thereby, the points on the same axis are associated with each other in the interference fringe data respectively acquired by the first optical interferometer 200 and the second optical interferometer 300. Further, the gap (d0) change between the first wire grid 220 and the second wire grid 320 is calculated by interference fringes.

Next, the workpiece W is set in the measurement mode, and the workpiece front surface data Da and the workpiece back surface data Db are acquired by the first optical interferometer 200 and the second optical interferometer 300, respectively.
Here, the workpiece front surface data Da represents a change in the distance (d1) between the first wire grid 220 and the workpiece front surface Sa.
The workpiece back surface data Db represents a change in the distance (d2) between the second wire grid 320 and the workpiece back surface Sb. Then, when the thickness change of the workpiece W is calculated by synthesizing the workpiece front surface data Da and the workpiece back surface data Db, the workpiece front surface data Da and the workpiece back surface data Db include the front and back surfaces of the workpiece. Note that the shape of the first wire grid 220 or the second wire grid 320 is included in addition to the shape. That is, the coaxial distance change (thickness change) TV between the shape profile Sa of the work front surface and the shape profile Sb of the work back surface is expressed by the following equation.

  TV = d0− (d1 + d2) (Formula 1)

  The calculated thickness change is output by a predetermined output means such as a monitor or a printer.

According to 1st Embodiment provided with such a structure, there can exist the following effects.
(1) In the calibration mode in which the workpiece W is not set, since the interference fringes acquired by the first optical interferometer 200 and the second optical interferometer 300 are the same, the first optical interferometer 200 and the second optical interferometer 300 are the same. By associating the same points in the data acquired in (1) and (2), the coaxial points on the data can be associated. The thickness change of the workpiece W can be obtained by synthesizing the workpiece front surface data Da and the workpiece back surface data Db in consideration of the correspondence between the already determined points on the same axis. That is, after associating points on the same axis in the calibration mode, by combining the workpiece front surface data Da and the workpiece back surface data Db obtained in the measurement mode, the workpiece thickness change can be accurately determined. Can be sought.

(2) In the calibration mode in which the workpiece W is not set, interference fringes in which the reflected light from the reference surface 223 of the first optical interferometer 200 and the reflected light from the reference surface 323 of the second optical interferometer 300 interfere with each other can be obtained. . Therefore, the unevenness difference between the two reference surfaces 223 and 323 can be obtained from the interference fringes acquired in the calibration mode. Therefore, after combining the workpiece front surface data Da and the workpiece back surface data Db, the thickness difference of the workpiece W can be obtained with high accuracy by further reducing the unevenness difference between the two reference surfaces 223 and 323. it can. Regardless of the difference between the unevenness of the reference surface 223 of the first optical interferometer 200 and the unevenness of the reference surface 323 of the second optical interferometer 300, the thickness change of the workpiece W can be obtained with high accuracy.

(3) Since the polarization axis directions of the wire grids 220 and 320 are orthogonal to each other between the first optical interferometer 200 and the second optical interferometer 300, light that has passed through the wire grid of one of the optical interferometers in the calibration mode. Is reflected by the wire grid of the other optical interferometer. Thereby, in the state which does not set the workpiece | work W, in both the 1st optical interferometer 200 and the 2nd optical interferometer 300, the interference fringe by the reflected light from two wire grids can be acquired similarly.

(4) Since the same light from the same light source 211 is used in the first optical interferometer 200 and the second optical interferometer 300, light having the same wavelength is used in the first optical interferometer 200 and the second optical interferometer 300. Can be used. Then, by using light of the same wavelength, the same interference fringes can be acquired in the first optical interferometer 200 and the second optical interferometer 300 in the calibration mode. Therefore, by associating the same points of the same interference fringes in the first optical interferometer 200 and the second optical interferometer 300, the coaxial point between the data in the first optical interferometer 200 and the second optical interferometer 300 is obtained. It can be accurately associated.

(Modification 1)
In the first embodiment, the interference fringe acquisition unit includes the polarizing plate that causes the P wave and the S wave to interfere and the CCD camera that images the interference fringe. However, the interference fringe acquisition unit is different at the same time. A phase shift interference fringe acquisition unit that acquires interference fringes of a phase difference may be used.
As a phase shift interference fringe acquisition part, the structure shown in FIG. 4 is mentioned as an example, for example.
In FIG. 4, the phase shift interference fringe acquisition unit 400 includes a quarter wavelength plate 410, a composite polarizing plate 420, and a composite CCD camera 430 as an imaging unit.
The quarter-wave plate 410 converts the S-wave reference light and the P-wave object light incident on the phase shift interference fringe acquisition unit into circularly polarized light having different rotation directions.
The composite polarizing plate 420 is a polarizing plate in which four polarizing plates 421 to 424 having different transmission axis angles are formed as a single plate, and is disposed in the optical path so that four portions of the light beam are combined. The plate 420 passes through polarizing plate portions 421 to 424 having different transmission axis angles.
The four polarizing plates 421 to 424 constituting the composite polarizing plate 420 have different transmission axis angles by 45 degrees, and the polarizing axes of the polarizing plates 421 to 424 are 0 degree, 45 degrees, 90 degrees, and 135 degrees. The composite CCD camera 430 includes four CCD cameras 431 to 434 corresponding to the respective polarizing plates 421 to 424 constituting the composite polarizing plate 420.

  For example, taking the first optical interferometer 200 as an example, the optical path until the composite CCD camera 431 captures the reference light that is an S wave and the object light that is a P wave will be described.

The light (S wave, P wave) that has passed through the beam splitter 240 is, for example, made into four parallel light beams by a beam branching unit (not shown), and then enters the quarter wavelength plate 410. Then, the P-wave object light and the S-wave reference light pass through the quarter-wave plate 410 and become circularly polarized light having opposite rotation directions. Then, a circularly polarized non-interfering light beam whose rotation direction is opposite is incident on the composite polarizing plate 420, and four portions of the light beam pass through the polarizing plate portions 421 to 424 of the composite polarizing plate 420, respectively.
When the non-interfering light beam passes through each of the polarizing plate portions 421 to 424, the reference light Lb and the object light La included in the non-interfering light beam interfere with each other to generate an interference fringe image. At this time, the transmission axis angles of the polarizing plates 421 to 424 constituting the composite polarizing plate 420 are different from each other by 45 degrees, and four interference fringes having phases different from each other by 90 degrees are generated. The interference fringe image enters the composite CCD camera 430 and is captured by the cameras 431 to 434 of the composite CCD camera 430. Then, as shown in FIG. 5, the luminous flux is divided into four, and interference fringes having different phases are obtained in the respective divided portions.

  Thus, when the interference fringes having different phases by 90 degrees are obtained, the interference fringes can be obtained by the calibration mode and the measurement mode, and the change in the workpiece thickness can be obtained.

(Modification 2)
As a phase shift interference fringe acquisition part, the structure shown by FIG. 6 may be sufficient, for example.
In FIG. 6, the phase shift interference fringe acquisition unit 500 includes a ¼ wavelength plate 510, an imaging lens 520, a non-interference light beam splitting prism 530, first to third polarizing plates 541 to 543, To third CCD cameras 551 to 553.
The non-interfering light beam splitting prism 530 is configured by bonding a first triangular prism 531, a second triangular prism 532, and a trapezoidal prism 533. The incoherent beam splitting prism 530 splits the incoherent light into three beams.
Here, the bonding surfaces of the prisms 531 to 533 are semi-transmissive surfaces.

The first to third polarizing plates 541 to 543 are inserted in the optical path of each light beam emitted from the non-interfering light beam splitting prism 530. The transmission axis angles of the first to third polarizing plates 541 to 543 are different by 60 degrees. When the transmission axis angle of the first polarizing film 541 is 0 degree, the transmission axis angle of the second polarizing film 542 is 60 degrees. The transmission axis angle of the third polarizing plate 543 is 120 degrees. The first to third light beams generated by the light beam splitting by the non-interfering light beam splitting prism 530 pass through the first to third polarizing plates 541 to 543, respectively, so that the object light La and the reference light in each light beam. Lb interferes with a different phase, and interference fringes with different phases are generated.
The first to third CCD cameras 551 to 553 are arranged at positions for imaging the interference fringes transmitted through the polarizing plates 541 to 543.

  When interference fringes having a phase difference of 60 degrees are obtained by the interference fringe obtaining unit 500, the interference fringes can be obtained by the calibration mode and the measurement mode, and the change in the workpiece thickness can be obtained.

(Modification 3)
As a phase shift interference fringe acquisition part, the structure shown by FIG. 7 may be sufficient, for example.
In FIG. 7, the phase shift interference fringe acquisition unit 600 includes a quarter wavelength plate 610, half mirrors 621 and 622 and a reflection mirror 623 that split a light beam into three, and a polarizing plate 631 disposed in each optical path. ˜633 and CCD cameras 641 to 643.
Reference light Lb and object light La having polarization directions orthogonal to each other pass through quarter-wave plate 610 as a non-interfering light beam. By passing the incoherent light beam through the quarter-wave plate 610, circularly polarized light in which the object light La and the reference light Lb having vibration directions orthogonal to each other included in the incoherent light beam are in rotation directions opposite to each other is obtained. Become.
The light that has passed through the quarter-wave plate 610 is divided into three light beams by the first half mirror 621, the second half mirror 622, and the reflection mirror 623 disposed on the optical path. Polarizing plates 631, 632, and 633 and CCD cameras 641, 642, and 643 are disposed on the optical paths of the divided light beams.

Here, the first polarizing plate 631 disposed on the optical path of the first light beam reflected by the first half mirror 621 and the optical path of the second light beam reflected by the second half mirror 622 are disposed. The second polarizing plate 632 and the third polarizing plate 633 disposed on the optical path of the third light beam reflected by the reflecting mirror 623 have different transmission axis angles.
For example, if the transmission axis angle of the first polarizer 631 is 0 degree, the transmission axis angle of the second polarizer 632 is 45 degrees, and the transmission axis angle of the third polarizer 633 is 90 degrees. Then, three interference fringe images having different phases by 90 degrees are captured by the CCD cameras 641 to 643. Then, the image of each interference fringe is input to a predetermined analysis means, and the image intensity at each point on the interference fringe is compared with the three interference fringes to obtain phase information on the workpiece surface. Thereby, the shape of the workpiece surface is required.

  Thus, when the interference fringes having different phases by 90 degrees are obtained, the interference fringes can be obtained by the calibration mode and the measurement mode, and the change in the workpiece thickness can be obtained.

It should be noted that the present invention is not limited to the above-described embodiment, and modifications, improvements, and the like within the scope that can achieve the object of the present invention are included in the present invention.
As the polarization separation means, the case where a wire grid is used has been described as an example. However, for example, a quarter wavelength plate may be used. As the light emitting unit, the case where the first optical interferometer and the second optical interferometer share one light source has been described. However, the first optical interferometer and the second optical interferometer have separate light sources. May be. In this case, it is preferable to match the wavelength of the light source between the first optical interferometer and the second optical interferometer. In this way, if the wavelengths of light used in the first optical interferometer and the second optical interferometer are matched, the interference fringes acquired by the first optical interferometer and the second optical interferometer in the calibration mode are the same. Become.

  The present invention can be used in an optical interferometer that measures the thickness of a workpiece.

The perspective view which shows the structure of an optical interferometer in 1st Embodiment of this invention. In 1st Embodiment, the side view of the optical interferometer of the state measured in a measurement mode. The side view of the optical interferometer of the state measured in calibration mode in 1st Embodiment. The figure which shows the structure of an interference fringe acquisition part in the modification 1. FIG. The figure which shows an example of the phase shift interference fringe acquired in the interference fringe acquisition part of the modification 1. The figure which shows the structure of an interference fringe acquisition part in the modification 2. In the modification 3, the figure which shows the structure of an interference fringe acquisition part. The figure which shows a Michelson type interferometer as a conventional optical interferometer. The figure which shows a mode that the front surface data and back surface data of a workpiece | work are matched. The figure which has arrange | positioned the conventional optical interferometer on the front and back of a workpiece | work.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Michelson type optical interferometer, 11 ... Light source, 12 ... Collimating lens system, 13 ... Half mirror, 14 ... Lens, 15 ... Interference fringe acquisition part, 16 ... Beam splitter, 17 ... Reference mirror, 100 ... Optical interferometer , 200 ... first optical interferometer, 210 ... light emitting section, 211 ... light source, 212 ... optical fiber, 213 ... opening end, 214 ... polarizing plate, 215 ... half-wave plate, 220 ... (polarization separating means) wire grid, 221 DESCRIPTION OF SYMBOLS ... Light transmission board | plate material, 222 ... Wire, 223 ... Reference surface, 230 ... Interference fringe acquisition part, 240 ... Beam splitter, 251 ... Collimator lens, 252 ... Collimator lens, 300 ... 2nd optical interferometer, 310 ... Lens, 320 ... Wire grid, 322 ... Wire, 323 ... Reference plane, 330 ... Interference fringe acquisition unit, 400 ... Phase shift interference fringe acquisition unit, DESCRIPTION OF SYMBOLS 10 ... 1/4 wavelength plate, 420 ... Composite polarizing plate, 430 ... Composite CCD camera, 500 ... Interference fringe acquisition part, 510 ... 1/4 wavelength plate, 520 ... Imaging lens, 530 ... Non-interference light beam splitting prism, 531 ... first triangular prism, 532 ... second triangular prism, 533 ... trapezoidal prism, 541 ... polarizing plate, 542 ... polarizing plate, 543 ... polarizing plate, 600 ... phase shift interference fringe acquisition unit, 610 ... wavelength plate, 621 ... Half mirror, 622 ... half mirror, 623 ... reflection mirror, 631 ... polarizing plate, 632 ... polarizing plate, 633 ... polarizing plate.

Claims (4)

An optical interferometer that measures changes in the thickness of a workpiece,
A first optical interferometer disposed on the front side of the workpiece;
A second optical interferometer disposed on the back side of the workpiece,
The first optical interferometer and the second optical interferometer are:
A light emitting part,
Polarization separating means having a reference surface disposed between the light emitting unit and the work and making the polarization direction of the reference light, which is reflected light from the reference surface, and the object light, which is reflected light from the work, at right angles When,
An interference fringe acquisition unit that acquires an interference fringe obtained by causing the reference light and the object light to interfere with each other;
The polarization separation means of the first optical interferometer and the polarization separation means of the second optical interferometer are arranged with the polarization axes being non-parallel,
When the workpiece is not placed,
The reference surface of the polarization separation means of the first optical interferometer reflects the light from the second optical interferometer to generate object light;
The optical interferometer, wherein the reference surface of the polarization separation means of the second optical interferometer reflects the light from the first optical interferometer to generate object light. The optical interferometer according to claim 1.
The polarization separation means is a wire grid type polarizing plate having a plurality of wires arranged in parallel to each other and reflecting a component parallel to the wire and transmitting a component perpendicular to the wire,
In the first optical interferometer and the second optical interferometer, the wire arrangement direction of the wire grid polarizer is non-parallel. The optical interferometer. The optical interferometer according to claim 1 or 2,
The optical interferometer, wherein the polarization separation means of the first optical interferometer and the polarization separation means of the second optical interferometer are arranged in a state in which polarization axes are orthogonal to each other. The optical interferometer according to any one of claims 1 to 3,
The light emitting unit of the first optical interferometer and the light emitting unit of the second optical interferometer are:
One light source to share,
An optical interferometer, comprising: light guide means for branching light from the one light source and guiding the light to the respective optical paths of the first optical interferometer and the second optical interferometer.

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